Ground fault protective devices used in a permanently connected installation (such as a wall box) require only that the hot or "live" conductor be interrupted in the event of a ground fault. However, in portable power equipment employing ground fault protection devices, the neutral conductor could become disconnected due to rough handling and the like and the protective circuit de-energized. A fault current between the hot conductor and ground would then go undetected through loss of the neutral conductor. This possibility has prompted U.L. to require that nonpermanently connected ground fault protective equipment in this country include means for detecting an open neutral conductor.
The means usually employed includes a separate device such as a self-latching relay that will maintain low current continuity only so long as both hot and neutral conductors are connected. These relay devices tend to be bulky and expensive and awkward for use with portable equipment.
Other known resettable circuit interrupt devices typically utilize complicated mechanisms to set and reset the contacts. These mechanical arrangements have tended to be complex and intricate because one necessary safety requirement is that the device must not be capable of being reset unless a safe circuit is present. This complexity, however, drives up manufacturing costs and reduces reliability.
Accordingly, the need exists for a resettable circuit interrupter that is simple in design, avoids complex mechanisms for actuating circuit connections and thereby provides a low cost yet reliable means for setting and resetting the device.
The apparatus disclosed in U.S. Pat. No. 4,893,101 provides such a device. While the device disclosed therein functions well, it relies solely on the absence or presence of a magnetic field provided by a predetermined minimum current in the solenoid coil. Therefore, a constant current is required through the coil to maintain the latching of the contacts in the closed position. Other methods exist in which a first current is used to close the contacts and then a second minimum current is utilized to maintain the contacts in the closed position. However, in both cases, at least a minimum current is required to hold the contacts in the closed position.
The present invention provides a device that is used as a circuit interrupter but which utilizes a magnetic coupling to hold the contacts in the closed position and has no power consumption except during the time the contacts are disengaged which, of course, lasts for only a few milliseconds. Further, the contacts will remain closed even if the power is removed and thus the device is unaffected by loss of power. However, appropriate circuitry may be added that causes the contacts to be opened in the event power is removed. The solenoid contacts may be either mechanically or electrically reset. The device can be used with extremely wide ranges of voltage without changing windings. Further, the solenoid and contacts are capable of conducting large amounts of current and yet are easily mounted on a printed circuit board.
Since the operation of the device relies solely on the presence of magnetic coupling, no mechanical or electrical latching arrangement is necessary. Therefore, minimum power is required to operate the device.
In another embodiment, minimum current is applied to the solenoid coil to create a magnetic field that is added to the magnetic field caused by the permanent magnet footplate to hold the contacts closed. In this embodiment, the conjunction of the magnetic field caused by the magnetic footplate with the magnetic field caused by the minimum current allows a much less current to be used in the bobbin than in the case such as disclosed in U.S. Pat. No. 4,893,101.
In still another embodiment, a standoff platen is added to the solenoid to prevent the contacts from being closed manually when at reset to prevent the solenoid from having chatter or buzzing.